Measuring and control apparatus



H. S. JONES MEASURING AND CONTROL APPARATUS 2 Sheets-Sheet; 1

2 F led Nov. 19, 1942 III! INVENTOR. HARRY s JONES ATTORNEY 8, 1944. s, JONES 2,355,537

MEASURING AND CONTROL APPARATUS I Filed NOV. 19, 1942 2 Sheetg-Sh e't 2 FIG. 4.

FIG. 5.

FIG. 6.

INVENTOR. HARRY S. JONES Patented Aug. 8, 1944 MEASURING AND CONTROL APPARATUS Harry S. Jones, Washington, D. (1., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application November 19, 1942, Serial No. 466,130

28 Claims.

The present invention relates to the measurement of minute electrical currents or potentials and their utilization for control purp and has particular utility in connection with the accurate measurement of electromotive forces having very low frequency components. Such electromotive forces, by way of example, may comprise the gradual and slow fluctuations in electromotive forces produced by a thermocouple and have been exceedingly difiicult of direct ampliflcation by the prior art arrangements.

An object of the invention is to provide a rugged and highly accurate instrument for indicating and/or recording minute unidirectional current or potential variations having low frequency components and/or for utilizing such currents or potentials for control purposes.

Another object of the invention is to provide an improved construction and arrangement of 5 parts whereby minute unidirectional currents flowing in low resistance circuits may be accurately measured.

Another object of the invention is to provide an improved method of eliminating the effects of stray electrical fields or spurious electrical effects upon the operation of apparatus designed for measuring the magnitude and the changes in magnitude of minute electrical currents or potentials.

A further object of the invention is to provide plifying systems is exceedingly difilcult becauseof the fact that changes in the spacing and position of the electrodes of electronic amplifying tubes produce eifects which are similar to and are of the same order of magnitude as the changes in the minute unidirectional potential orcurrent variations to be measured.

It has been proposed in the prior art to amplify such minute potential or current variations in low resistance circuits by periodically interrupting the low resistance circuit and utilizing the resulting pulsating potential or current to produce an alternating current of one phase or of opposite phase depending-upon the polarity of the unidirectional potential or current under measurement. In addition it 'has been proposed to periodically interrupt said low resistance circuit at the frequency of a given source of alternating current, to impress the resultant current or its alternating current component on the input circuit of an electronic amplifier, and to apply the output current of the amplifier to a reversible electrical motorto thereby selectively energize the motor for rotation in one direction or the other in accordance with the polarity of the unidirectional potential or current under measurement. In such prior art arrangements the motor controls the operation of various devices such, for example, as a potentiometer recorder. When the. minute unidirectional potential to be measured is that produced by a thermocouple which is subjected to the temperature within a furnace, the motor may aso be employed to control the operation of a valve for controlling the fuel supply, draft, or other agent for establishing the desired temperature condi tion within the furnace.

In such prior art arrangements for measuring the magnitude of minute unidirectional potential or current variations in low resistance circuits, a serious objection encountered has been the introduction of error into the measurement ob- 1 tially eliminated.

A more specific object of the invention is to provide in such measuring apparatus an arrangement including a suitable circuit interrupting device for translating the minute unidirectional potentials or currents in the low resistance circuit into an alternating current which may be readily amplified and in which the frequency of operation of the circuit interrupting device is not critical and need not remain constant.

Another specific object of the invention is to provide in such measuring apparatus an arrangement for translating minute unidirectional potential or current variations into alternating currents which may be readily amplified in which the use of iron core transformers is not required and air core transformers may be utilized instead to the" end that the induction of extraneous fluctuating or alternating currents into the measuring circuit from stray electrical or magnetic fields is reduced to a minimum.

A still further specific object of the invention is to provide in such measuring apparatus an arrangement for translating the minute unidirectional potential or current variations in low which may be readily amplified in which the frequency of operation of the circuit interrupting device is a relatively high frequency compared to the frequency of ordinary commercial alternating current supply sources, and the derived high frequency alternating current is employed to control the actuation of motive structure whereby the effects of extraneously induced alternating currents of commercial or other relatively low frequencies into the circuit are rendered incapable of affecting the operation of the apparatus.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a diagrammatic representation of the use of the invention in a self balancing potentiometric recording system; and

Fig. 2 illustrates in detail a form of current interrupter device that may be employed in the conversion circuit of the arrangement of Fig. 1;

Fig. 3 shows curves explaining the operation of the conversion circuit of Fig. 1; and

Fig. 4, 5 and 6 illustrate modifications of the conversion circuit of Fig. 1.

In Fig. 1 of the drawings there is illustrated in schematic form an arrangement including electronic amplifying means for producing effects in accordance with the extent of unbalance of a potentiometric network I which controls the electronic amplifying means and is unbalanced in accordance with the variations in a minute unidirectional electromotive force to be measured, namely, that produced by thermocouple 2, and in which because of the small magnitude of the electromotive force under measurement it is not practicable nor desirable to have the said effects produced directly by said electromotive force.

Specifically, an arrangement is illustrated in Fig. 1 of the drawings for recording the temperature of a furnace 3 in the interior of which nace 3 may be located at a distance from the remainder of the apparatus and the thermocouple 2 has its terminals connected by a pair of conductors and 5 to the terminals of the potentiometric measuring network I. The potentiometric measuring network I includes a main slidewire resistance 5 and an associated contact 'I which is capable of being moved along the length of the slidewire 6, and may be of any suitable type, for example, such as the Brown potentiometric measuring network disclosed in the Harrison et al. Patent 2,150,502 issued March 14, 1939.

More particularly, the potentiometric measuring network I is of the split potentiometer type and comprises three branches connected in parallel. one branch including the main slidewire resistor 6, a second branch including balancing resistances 8 and 9, and the third branch including a battery l8 or other source of current and a-resistance I! which may be adjusted as required to insure a current flow of the desired magnitude through the resistance 5. The sliding contact I engages the main slidewire 6 at the point I2 the position of which along the length of the resistor 6 is varied by the sliding adjustment of the contact I. The contact I as shown is a bridging contact connecting the point I2 of the resistor 6 to a corresponding point of a slidewire resistance I3 which is positioned alongside the resistor 5.

The resistor I3 has one end connected by the conductor 5 to one terminal of the thermocouple 2 and is employed for the purpose of avoiding measurement inaccuracies due to variations in resistance to the flow of current generated by the thermocouple 3 which would otherwise result from variations in the relative resistances of the portions of the resistor 5 at the opposite sides of the point I2. The second terminal of the thermocouple 2 is connected by means of the conductor 4 to one terminal of a circuit interrupting device I4 which is illustrated in detail in Fig. 2 and is described in detail hereinafter. The other terminal of the device I4 is connected to the branch of the potentiometric measuring network I including the resistances 8 and 9 at the point I5 in said branch intermediate said resistances through a circuit including the primary winding ll of an air core transformer I8, an inductance I9 and a filter I5, all of which elements are inserted in the conductor 4. Filter I6 is provided to prevent stray alternating currents which may be introduced into the potentiometric network I from flowing through the circuit branch including device I4, transformer primary winding I1 and inductance I9 and produces this desired result by providing a path between the potentiometric network point I5 and the thermocouple lead 4 which presents low impedance to such stray alternating currents but presents high impedance to the unbalanced potentiometric current flows and thus diverts the stray alternating currents from that circuit branch. A condenser 20 is connected in shunt to the primary winding IT. The constants of the transformer primary winding I! and condenser 20 are so chosen, as is explained more in detail hereafter, that when a unidirectional potential is periodically impressed on the parallel connected circuit including primary winding I l and condenser 20 a rapidly diminishing oscillatory discharge current flows through the local circuit including only these elements.

The flow and direction of fiow of unbalanced potentiometric current through the circuit branch from the point I5 intermediate the resistances 8 and 9 and including the filter IS, inductance I9, transformer primary winding I'I, device I4, conductor 4, thermocouple 2, conductor 5, and the portion of resistance I3 at the right of bridging contact 1 depends upon the relation between the electromotive force produced by the thermocouple 2 and the potential diiference between the potentiometric network points I2 and I5. The thermocouple 2 is so connected to the potentiometer circuit that the electromotive force of the thermocouple opposes the potenial difierence beween the network points I2 and I5. The potential difference between the points I2 and I5 is decreased and increased by movement of the contact I to the left and to the right, respectively. With a suitable adjustment of the contact I the potential difference between the points I2 and I5 will be equal and opposite to the electromotive force produced by the thermocouple 2 and no current will fiow through the above mentioned circuit branch including the thermocouple 2. On an increase in the thermocouple electromotive force above the and establishes a field in the rotor which is dis placed 90 in one direction or the other with respect to that established therein by the winding 10. Reaction between the field set up by the potential difference between the points 12 and 6 winding 30 with that set up by winding 28 estab- IS a resultant current will fiow in one direction through the interrupter I4 and transformer primary winding l1 and such current fiow may then be eliminated by a suitable adjustment of the bridging contact 1 to the right. Such adjustment increases the potential diflerence produced between the potentiometric network points l2 and I5 and opposed to the thermocouple electromotive force and when of proper magnitude reduces the resultant current flow to zero. Conversely, when the electromotive force of the thermocouple 2 falls below ,the potential difierence between the potentiometer points I! and II the resultant cur rent flow through the interrupting device I and transformer primary winding I! will be in such direction that it may be eliminated by a suitable adjustment of the bridging contact I to the left. Adjustment of contact I to the left decreases the potential difierence opposed to the thermocouple electromotive force, and when of proper magnitude, decreases to zero the resultant current fiow.

As is illustrated more or less diagrammatically in the drawings, the bridging contact I is ad justed along the slidewire resistances I and II by the operation of a reversible electrical motor 2| which is shown as having its rotor mechanically coupled to a threaded shaft 22 on which a carriage 23 which supports the bridging contact I2 is mounted. 'Thus, the bridging contact I2 is adjusted longitudinally of the shaft 22 in one direction or the other according to the direction of rotation oi the reversible electrical motor 2|. The reversible electrical motor 2| has terminals 24 and 25 connected to the output terminals oi! the electronic amplifying means referred to above and has other terminals 28 and 21 connected to alternating current supply conductors L and L which also supplies energizing current to the electronic amplifying means.

If desired, a pen may be mounted on the carriage 23 which carries the potentiometer bridg ing contact 1 and arranged in cooperative relation with a recorder chart 28 to thereby provide a continuous record of the temperature of the interior of the furnace 3. The chart 28 may be a strip chart as shown and is adapted to be driven in any convenient manner as, for example, by a unidirectional electrical motor (not shown) through suitable scaring so that a record of the temperature to which the thermocouple 2 is subjected will be recorded as a continuous line on the chart.

For its intended use the motor 20 may be of the form diagrammatically shown in the drawings and comprising a rotor, two pairs 01' oppositely disposed field poles (not shown) 'onone pair of which a winding 29 is wound and on the other pair of which a winding Iii is wound. Winding 30 is connected to the terminals 26 and 21 of the motor and is supplied with energizing current from the alternating current supply conductors L and L through a condenser 3| of suitable value. Due to the action oi the condenser II. the current which fiows through the winding 30 will lead-the 'line current by approximately 90.

Pulsating current is supplied the winding 29 by the electronic amplifying means reierred to andthis current is in phase with or is displaced 180 in phase with respect to the supply line voltage I lishes a rotating field in the rotor which rotates in one direction or the other depending upon whether the winding 2! is energized with current in phase with the supply line volta e or displaced 180 in phase therewith, and thereby, as is explained in detail-hereinafter, in accordance with the direction of unbalance of the potentiometric measuring network I. The direction and duration of rotation of the motor is controlled in accordance with the direction and extent of unbalance of the potentiometric measuring network I so that on motor rotation the bridging contact 1 is adjusted in the proper direction to reduce the potentiometric unbalance.

The circuit interrupting device 14, shown schematically in Fig. 1 is illustrated in more detail in Fig. 2 and is utilized for periodically impressing the potentiometric unbalanced unidirectional potential on the condenser 20 to create a succession of rapidly vanishing oscillatory discharge current waves in the local circuit including the condenser 20 and the transformer primary winding I! which are capable of being readily amplified. In Fig. '3 the full line curve A illustrates the oscillatory discharge current flow in winding l'l when the potential impressed on condenser 20, is of one polarity and the dotted curve B shows the oscillatory discharge current flow when the potential impressed on condenser 20 is of opposite polarity. The manner in which these oscillatory discharge current flows are utiliztd to control the rotation and direction of rotation of motor H is explained hereinafter. It will be understood that any desired form of interrupter, for example, a reed type interrupter or vibrator, having the same operation characteristics as the interrupter I4 shown in detail in Fig. 2 may be utilized, if desired. I

The circuit interrupting device It disclosed in Fig. 2 includes a vacuum tube 32 in which metal contacts 33 and H are arranged. The'tube 32 is preferably formed of glass and has one end resiliently formed integral with a rod 35 the right end 38 of which normally rests against the underside of the metal contact 34 so that upon movement of the left end of rod 35 downward, the contact 34 is raised out of engagement with the contact 33 to break the circuit at the point 31. As soon as the rod 35 is released the contact .3 will again move into engagement with contact 33 to close the circuit. 7

The rod may be raised and lowered in any convenient manner and by way of example a radio loudspeaker element 38 may be employed for this purpose. The loudspeaker element 38 is connected by means of a link 39 to the rod 35 whereby the rod 35 will be'rapidly raised and lowered, as the loudspeaker vibrates, thereby making and breaking the circuit at point 31.

In order to operate the loudspeaker element at the desired high frequency there is provided an electron discharge device 40 having anode, grid, cathode and heater fi ament elements. The

electron discharge device it is electrically co'n-- the grid or input circuit and the plate or output circuit of device 43. This interlinkage is an inductive one and is obtained by means of two inductively associated windings or coils 4| and 42 which are connected respectively in the grid and anode circuits.

It will be understood that any suitable electrical energizing connections for the electron discharge device 40 may be used, and in Fig. 2 it is energized from a suitable source of alternating voltage. When the electron discharge device 48 has a line voltage filament, it may be energized directly from the alternating current supply conductors L and L as shown. The supply conductor L is connected by a conductor 53, in which-an inductive choke 44 is inserted, to the anode and to one terminal of both the heater filament and the inductive coil 42. The supply conductor L is connected by a conductor to the other terminal of the heater filament,

to the other terminal coil #2, to the cathode of device W, and to one end of inductive coil 45. It will be observed that the heater filament and inductive choke 44 are connected in series across the supply conductors L and L The characteristics of the electron discharge device 40, the voltage of the supply conductor L and L and the inductive choke 44 are so related with respect to each other that the desired anode voltage is obtained and also that the desired current flows through the heater filament.

In accordance with the present invention the electrical constants of the transformer primary winding 3? and the condenser are so related to each other as to satisfy the condition of R 4 L/C where R is the resistance in ohms of the local circuit including the elements l! and L s the inductance in henries of the transformer primary winding l7. and C is the capacitance in iarads of the condenser 28. it can be shown mathematically that when this condition is satisfied a rapidly vanishingoscillatory discharge current will flow through the local circuit including elements l! and 2G whenever a unidirectional potential is impressed on the parallel connected winding i"! and condenser 23. For example, see pages 1&0 and 141 of the text An Elementary Treatise on Differential Equaticns. (2d ed. 1933) by Abraham Cohen and publ shed by D. C. Heath and Company. The desired frequency of oscillation of this discharge current flow be obtained by proper selection of the circuit constants R, L and C. It will be noted that he frequency of oscillation may be a very high frequency, for example of the order of radio frequencies.

The production of such an oscillatory discharge current flow through the transformer primary winding l? and condenser 20 will occur during each engagement and disengagement of the interrupter contacts 33 and 34 so long as such engagement and disengagement is operative to effect a change in the potential impressed on the parallel connected. elements i"! and 20. That is to say, when the potentiometric network I is unbalanced in the d rection to render the network point positive with respect to the networl; point i2, closure of the interrupter con tacts and 3 operates to impress this potential. on the parallel connected winding H and con denser and thereby tends to render the upper terminal of the parallel connection positive with respect to the lower terminal. The electrical character stics of the parallel connected elements I? and 20 are such that they tend to resist changes in electric potential thereacross and as a result the potential across these elemerits is not instantaneously changed an amount corresponding to the difference in potential be tween the potentiometric network points l2 and I5. Instead the potential change is gradual and is accompanied by an oscillatory discharge between winding l1 and condenser 20 which gradually vanishes as the potential across the parallel connected elements approaches the potential difference between the potentiometric network points 12 and l5. Such decay of the oscillatory discharge is completed during the time that the contacts 33 and 34 of interrupter l4 are closed, as is illustrated in Fig. 3.

In Fig. 3 the distance between vertical lines C and D indicates the time during which the contacts 33 and 34 are in engagement. It will be noted that during this time the oscillating wave A (which for purposes of illustration is presumed to be created for the condition of potentiometric unbalance considered, namely when the potentiometric network point I5 is positive with respect to the point 12) diminishes and becomes practically negligible within the time C--D.

When the contacts 33 and 34 of the interrupter N are opened to disconnect the unbalance potentiometric potential from winding I! and condenser 20, a similar oscillatory discharge occurs in the local circuit including elements I! and 20. This oscillatory discharge gradually diminishes as the energy stored in the condenser 20 and winding IT dissipates itself and becomes practically negligible dur ng the time that the contacts 33 and 34 are disengaged. Since the space between vertical lines D and E in Fig. 3 illustrates the time during which the contacts 33 and 34 are disengaged, it will be noted that the oscillating wave A is diminished before the contacts 33 and 34 are again closed.

It will observed that for the direction of potenticmetric network unbalance considered the successive oscillating current waves established in the transformer primary winding H and condenser 20 commence at a maximum positive value irrespective of whether that oscillating wave is created as a result of engagement or disengagement of the interrupter contacts 33 and 34. When the potentiometric network I is unbalanced in the opposite direction to render the network point l2 positive with respect to the point i5 and thus cause a unidirectional potential of opposite polarity to be impressed on" the parallel connected winding IT and condenser 2!! whenever the interrupter contacts 33 and 34 are closed, however. the successive oscillating waves created upon successive engagements and disengagements of contacts 33 and 34 and indicated by the reference character B, commence at a maximum negative value. Thus. the oscillating Waves created for the second mentioned direction of potentiometric unbalance are exactly out of phase with the oscillating waves created for the first mentioned direction of potentioinetric unbalance. The frequency of the oscillating waves is the same in each case inasmuch as the circuit constants of the local oscillating circuit l1 and 20 are the same irrespective of the direction of potentiometric unbalance.

When the potentiometric netwo k is ex ctly balanced, and therefore, no potenti l is impressed across the parallel connected winding l1 and con denser 2U, operation of the interrupter I4 is ineffective to cause an oscillatory discharge in the a transformer I8.

The oscillating current flow established in the transformer primary winding I1 upon unbalance of the potentiometrlc network I in one direction or the other produces'a corresponding oscillating current flow in the secondary winding 46 of the To facilitate the induction of such an oscillating current flow in secondary winding 46, the latter is tuned by a condenser 41 to the same frequency to which the circuit including elements l1 and is tuned.

This oscillating potential produced across the transformer secondary winding 46 is amplified by a suitable electronic amplifier 48, the output terminals of which are connected through a condenser 49 to a resistor 50. As a result an oscillating potential is produced across the resistance 50 which corresponds to the oscillating potential induced in the transformer secondary winding 46 but is greatly amplified with respect to the latter.

The oscillating potential thus produced across the resistance 50 is applied by conductors 5| and 52 to the input circuit of a heater type electronc valve 53. The electronic valve 53 is a twin type amplifier valve and includes two triodes in the same envelope. One of the triodes includes an anode 54, a control electrode 55, a cathode 55 and a heater filament 51, and the other of the triodes includes an anode 58, the control electrode 59, a cathode 60, and a heater filament 6|.

Transient oscillating anode voltage of the same frequency as that of the transient oscillating potential produced across the resistance 50 is applied to the electronic valve 53 from the secondary windings, 62 and 63 of an air core transformer 64 having a primary winding 65. More specifically, the transformer secondary winding 62 supplies a transient oscillating anode voltage to the triode including the anode 54 which is in phase with the transient oscillating potential produced across the resistor 50 upon unbalance of the potentiometer network in the direction to render the network point I5 positive with respect to the point I2, for example, through a circuit which may be traced from the cathode 56 to a conductor 66, a contact 61 which is slideable along a resistance 68, the portion of resistance 68 to the left of contact 61, as seen in the drawlugs, and which is shunted by a condenser 69, the transformer secondary winding 62 and a conductor 69a to the anode 54. A transient oscillating anode voltage which is displaced 180 in phase with respect to the voltage applied to the anode 54 is supplied the triode including the anode 58 from the transformer secondary winding 63 through a circuit which may be traced from the cathode 60 to the conductor 66, contact 61; the portion of resistance 68 to the right of contact 61 and which is shunted by a condenser 1|, the transformer secondary winding 63 and a conductor 12 to the anode 58. It will be noted that the cathodes 56 and 60 of valve 53 are connected together as are also the control electrodes 55 and 59 thereof.

When an oscillating potential of one phase is produced across the resistance 50 one triode or the other of valve 53 will be rendered more conductive than the other to thereby produce a potential drop across the portion of resistance 68 to the left of contact 61 which is different from that produced across the portion of resistance 68 to the right of contact 61. The condensers 69 and 1| are provided across the portions of resistance 68 to the left and right of contact 61, respectively, for the purpose of smoothing out the ripple in the potential drops produced across these resistance'portions by the flow of current therethrough from the electronic valve 53.

In accordance with the present invention anode voltage for the electronic valve 53 is derived from an oscillating circuit in which oscillatory dis-.

charge current flows are established under control of the operation of the interrupter switch I4. This oscillating circuit includes the primary winding 11 of a transformer 16 and a condenser 19 which is connected in parallel to the winding I1. A variable portion of the potential drop which is maintained across a slide wire resistance 10 by a battery 14 is impressed on this oscillating circuit through a circuit which may be traced from the upper end of resistance 10 as seen in the drawings, interrupter switch l4, the parallel connected elements 11 and 19 and an inductance 13 to a contact 15 which is in slidable engagement with the resistance 15. Inductance 13 is provided to prevent the oscillatory discharge current in winding 11 and condenser 19 from flowing back through slide wire resistance 10 and battery 14. A rheostat is provided in circuit with the battery and the slide wire resistance 10 for facilitating the adjustment of the potential drop maintained across resistance 10'by battery 14 to the desired value. impressed on the parallel connected winding 11 and condenser 19 from the resistance 10 is somewhat greater than the largest unbalanced potential which is derived in the potentiometric network I.

The constants of the oscillating circuit including the transformer primary winding 11 and the condenser 19 are so chosen that whenever the contacting members 33 and 34 of the interrupter switch l4 are opened or closed, a rapidly vanishing oscillatory discharge current is established in the oscillating circuit. Further, the constants are so chosen that the frequency of oscillation of this discharge current is the same as the frequency of the oscillatory discharge current which is established in the oscillating circuit including transformer primary winding I1 and condenser 20 upon opening and closing of the contacting members 33 and 34 of switch I4. The oscillatory discharge current established in the oscillating circuit including elements 11 and 19 has been illustrated in Fig. 3 by the curve F. Since this latter oscillatory discharge current is established as a result of opening and closing of the same contacting members which establishes the oscillatory discharge current in the oscillating circuit including elements I1 and 20, namely, contacting members 33 and 34 of switch I4, the oscillatory discharge current established inthe circuit including members 11 and 19 will be either in phase with or out of phase with the oscillatory discharge current established in the circuit including elements I 1 and 20 depending upon the direction of unbalance of the potentiometric network I. As will be noted by reference to Fig. 3, an oscillatory discharge current is established in the oscillating circuit including winding 11 and condenser'19 upon each engagement and disengagement of the contacts 33 and 34 of the switch I4. In addition, the oscillatory discharge current which is so produced,

Preferably the voltage rapidly vanishes and becomes practically negligible during each interval of engagement or disengagement of the contact members 38 and 34.

The oscillatory discharge current flow thus established in the transformer primary winding 71 produces a corresponding oscillating potential in the secondary winding 18 which is provided on the transformer 18 in inductive relation to the Winding 11. The transformer secondary winding 18 is tuned by a condenser 88 to the frequency of the oscillating current flow in the winding 11.

The oscillating potential produced in the transformer secondary winding 18 is amplified by a suitable electronic amplifier 81 having a pair of input terminals to which the winding 18 is connected and having a pair of output terminals to which the primary winding 85 of transformer 84 is connected. It should be observed that since the polarity of the potential maintained across the slide wire resistance l by the battery 14 is constant, the phase of the transient oscillating potential introduced into the transformer secondary winding 18 and consequently in the secondary windings 62 and 83 of transformer 64 always remains constant. The secondary windings 82 and 63 are so connected to the output circuits of the triodes of valve 53 that the transient oscillating potential impressed on the anode 54 is exactly 180 out of phase with the 0scillating potential applied to the anode 58.

The electronic amplifiers 48 and 81 for amplifying the transient oscillating potentials established in the transformer secondary windings 48 and I8, respectively, may be of any suitable type and for example may be so-called intermediate frequency amplifiers. Direct current energizing voltage for the amplifiers 48 and 8| is derived from a suitable rectifier which is generally designated by the reference numeral 82 and which in turn is energized by alternating current derived from the alternating current supply conductors L and Ir".

Direct current energizing voltage for heating the filaments 51 and Si of valve 53 is also obtained from rectifier 82. The connections through which this voltage is applied to heater filaments 51 and 6! from rectifier 82 have not been shown in order not to confuse the drawings.

The potential drop produced across the portion of resistance 68 to the left of contact 51 is applied to the input circuit of one triode of a heater type electronic valve 83 having a second V triode on the input circuit of which the potential produced across the right hand portion of resistance 68 is applied. The first mentioned triode of the electronic valve 83 includes an anode 84, a control electrode 85, a cathode 86, and a heater type filament 81. The second-mentioned triode includes an anode 88, a control electrode 89, a cathode 88, and a heater type filament 9|. Energizing current is supplied the heater filaments 81 and Si, which are connected in parallel, from the low voltage secondary winding 82 of an iron core transformer 93 having a line voltage primary winding 84 and a high voltage secondary winding 85 in addition to the low voltage secondary winding 82.

Anode voltage is supplied the triode including the anode 84 from the transformer secondary winding 95 through a circuit which may be traced from the left end of the winding 85 to the anode 84, the cathode 88, and a conductor 88, in which the field winding 28 of motor 2! is connected, to a center tap on the transformer secondary winding 85. A tuning condenser 81 is connected across the motor field winding 88. Anode voltage is supplied to the triode including the anode 88 through a circuit which may be traced from the right end of the transformer secondary winding to the anode 88, cathode 88, conductor 86, and the motor field winding 88 to the center tap on the winding 85.

As shown, the cathodes 86 and 88 of the valve 88 are both connected to the contact 81 which is in engagement with resistance 68. One end of the resistance 88 is connected to the control electrode 85 of valve 83 and the other end of the resistance 68 is connected to the control electrode 88.

The motor driving circuit disclosed in Fig. i is disclosed and is being claimed in the Wills application Serial No. 421,173 which was filed December 1, 1941. Therefore, for the present purposes, it is believed sufficient to note that the motor 8! is preferably so constructed that the impedance of the winding 28 is of the proper value to match. the impedance of the anode circuits of the triodes of valve 83 when the motor is operating in order to obtain the most efficient operation. Preferably, the motor is so constructed that it has a high ratio of inductance to resistance, for example, of the order of from 6 to 1 to 8 to 1 at the frequency of the energizing current supplied to it. This provides for maximum power during the running condition of the motor with the least amount of heating and also provides a low impedance path for braking purposes.

As noted hereinbefore energizing current is supplied to the motor winding 38 from the alternating current supply conductors L and 1/ through the condenser 8|. The condenser II is so selected with respect to the motor winding 88 as to provide a series resonant circuit having a unity power factor. Due to the series resonant circuit, the total impedance of the circuit including winding 38 and condenser ii is substantially equal to the resistance of the winding 83 and since this resistance is relatively low, a large current fiow through the winding 88 is made possible. This permits the attainment of maximum power and torque from the motor I0. Due to the series resonant circuit, the current flow through the motor winding 38 is in phase with the voltage of the supply conductors L and L. The voltage across the motor winding 88, however, leads the current by substantially because of the inductance of the winding 38.

Energizing current is supplied the motor winding 28 through the anode circuits of the triodes of valve 83 which were previously traced. The condenser 81 connected in parallel with the winding 28 is so chosen as to provide a parallel circuit having a unity power factor. This parallel resonant circuit presents a relatively high external impedance and a relatively low local circuit impedance. The relatively high external impedance is approximately the same as the impedance of the anode circuits of the triodes of valve 88 and thereby provides efficient operation. The relatively low or internal circuit impedance approximates the actual resistance of the winding 88 and since this resistance is relatively low, the impedance of the local circuit is relatively low.

During the first half cycle of the alternating voltage produced across the terminals of the transformer secondary winding 85 the anode 84 of valve 83 is rendered positive with respect to the center tap on the winding 85 and during the second half cycle the anode 88 is rendered positive with respect to the center tap on the winding 95. Accordingly, the triodes of valve 83 are arranged to conduct on alternate half cycles.

For the condition when the potential drop across the portion of resistance 68 to the left contact 61 is the same as the potential drop across.

resistance 68 to the right of contact 81, the potentials of the control electrodes 95 of valve 81 will be exactly the same and therefore the conductivity of the triode including anode 84 will be the same'as the conductivity of the triode including anode 99. Pulses of current, of equal magnitude will then flow in the motor winding 29 from the anode circuits of the triodes of valve 93 during each succeeding half cycle of the alternating voltage supplied by the transformer secondary winding 95. Thus, for the'condition under consideration pulsating direct current of twice the frequency of the alternating voltage supplied by the transformer secondary winding 95 is supplied the motor winding 29. When thus energized, the motor 2I is not effectively urged to rotation in either direction and remains stationary. Due to the relatively high direct current component of the current then flowing through the motor winding 29 the core structure of the motor 2| tends to be saturated whereby the inductive reactance of the motor winding 29 is relatively small. The condenser 91 is so selected that the condenser in parallel with the motor winding 29 then forms a parallel resonant circuit with the latter. Such saturation of the core structure of motor 2I operates to exert an appreciable damping effect on the rotor of the motor, that is, an efiect tending to prohibit rotation of the motor. Thus, if the motor had been rotating, such saturation of the core structure will operate to quickly stop the motor rotation. When the potential drop across theportion of resistance 59 to the left of the contact 61 is rendered different from the potential drop across the portion of that resistance to the right of contact 61, the magnitude of the pulses of current flowing through the motor winding 29 from one triode of valve 83 will be increased and the magnitude of the pulses of current flowing through the winding 29 from the other triode will be decreased. When the motor field winding 29 isthus energized, the direct current component of the current flowing through the motor winding 29 is decreased whereby the saturation of the motor core structure and the consequent motor damping effect is reduced. In addition, the alternating component of the current supplied the winding 29 is increased. This alternating component produces an alternating field in the motor core structure which reacts with that established by the motor winding 39 to produce a rotating field in the motor. This rotating field rotates in one direction or the other depending upon the direction of potentiometric unbalar cc and effects actuation of the motor for rotation in a corresponding direction,

with the arrangement described, it will be noted that the potential drop maintained across the slide wire resistance 19 by the battery II also.

tential is established on the parallel connected elements I1 and 29 only during the intervals when the contact members 33 and 34 of the interrupter switch I4 are out of engagement with each other. During the intervals when these contact members are in engagement, the portion of the circuit just traced and including the potentiometric I, the thermocouple 2, filter I6 and the parallel connected elements I1 and 29 is shunted by the contact numbers 33 and 34 and therefore, during these intervals no potential is impressed on the parallel connected elements I1 and 29 from the slide wire resistance I9. The result of the potential impressed on the parallel connected elements I1 and 29 from the slide wire resistance I9 during the intervals at the contacts 33 and 34 are out of engagement is to establish an oscillatory discharge current flow through the primary winding I1 and condenser 29, irrespective of the state of balance of the potentiometric network I. Since the potential drop impressed across slide wire resistance 19 by battery 14 remains constant, the oscillatory discharge current fiow produced in the primary winding I1 and therefore the transient oscillating potential induced in the transformer secondary winding 46 also remains constant. Consequently, a constant transient oscillating current flow is established at the resistance 59 which operates to render one triode of the electronic valve more conductive than the other triode even when the potentiometric network I is precisely loalanced. If the contact 61 were then maintained half way between the ends of resistance $9, the potential drop across the left half of reslstance 69 would consequently be different from that across the right half. Under this condition the reversible motor 2I would be energized for rotation in one direction or the other. Such operation is undesired and in accordance with the present invention is obviated by adjusting the contact B'I along the slide wire resistance 63 so that the potential drop across the portion of resistance 68 to the left of contact 61 is exactly the same as the potential drop across the portion of resistance 69 to the right of the contact 51. When the contact 61 is so adjusted along the length of resistance 69 the potential of the control electrode 95 of valve 83 will be exactly the same as the potential control electrode 99 and as a result, the motor 2| will not be energized for rotation in either direction. If desired, a battery of suitable value maybe connected in the conducting lead from one or the other control electrode of valve 83 to the corresponding end of resistance 88 to provide a bias potential to achieve this result.

Upon unbalance of the ipotentiometric net work I in one direction or the other, a transient oscillatory discharge current will be established in the transformer primary winding I'l during the intervals when the contacts numbers 33 and 39 of interrupter switch I9 are in engagement and this oscillatory discharge current flow will operate to produce a corresponding transient oscillating potential across the resistance 59 which is effective to change the relative conductivities of the two triodes of valve 53 and thereby to eil'ect a change in the potential drop across the portion of resistance 68 to the left of contact 91 in comparison to the potential drop across the portion of resistance 68 to the right of contact I1. This will operate to produce a change in the potential of the control electrode of valve 93 rei'ativc to the potential at the control electrode 89, and therefore, cause the motor II to be energized for rotation in one direction or the other, depending upon the direction of potentiometric network unbalance. Thus, the undesired effect introduced into the operation of the apparatus of a potential being impressed across the operating circuit including elements I1 and 20 from the slide wire resistance I may be readily compensated for and cancelled out by suitable adjustment of the contact 61 along the slide wire resistance 60.

In the use of the invention in the form shown in Fig. l of the drawings it may also be desirable to eliminate effects due to stray alternating electromotive forces of high frequency which may be impressed on the potentiometric measuring circuit as a result of alternating current fiows in the alternating supply lines L and L or in other electrical apparatus which may be located near the potentiometric measuring network. Such disturbing effects may be eliminated by the use of the filter I6 which comprises suitable condensers and a resistance connected into the potentiometric measuring circuit as shown.

In the use of the present invention the filter I6 may not eliminate the undesired stray alternating electromotive forces to a sufficicnt degree, and in such case such undesirable electromotive forces may be substantially completely eliminated by means of an arrangement including an electronic amplifier 98 having input terminals 99 and I00 and output terminals IOI and I02. The input terminals of the amplifier 99 are connected across the output terminals I03 and I04 of the filter I5 and the output terminals of the amplifier 99 are connected across the input terminals I05 and I06 of the filter IS. The output terminals of the amplifier 98 are so connected to the input terminals of the filter I6 that the phase of the output voltage of the amplifier 98 is displaced 180 from the alternating voltage extraneously induced in the potentiometric measuring network I, and therefore, operates to neutralize or effectively cancel out the extraneously induced voltages which may leak through the filter IS.

The measuring circuit arrangement disclosed in Fig. l is characterized in that the frequency at which the circuit interrupter I4 operates to interrupt its associated circuit is not critical and need not be maintained constant since the frequencies of the transient oscillating voltages induced in the secondary windings 46 and 18 of the air core transformers I8 and 16 are dependent only on the frequency to which the oscillating circuit associated therewith is tuned. Furthermore, since the transformers I8 and 15 are air core transformers, the tendency of extraneous alternating currents to be induced directly in the input circuits of the amplifiers and BI is practically negligible.

It will be apparent that the reversible electrical motor 2| may be employed to operate a valve I01 positioned in a fuel supply pipe I08 to the furnace 3 'for varying the supply of heating agent to the furnace 3, or preferably, a separate reversible electrical motor may be so employed. For example, as illustrated in Fig. 1 of the drawings, a reversible electrical motor I09 having two opposed field windings (not shown) may be used for this purpose. The reversible motor I09 is mechanically connected in any S it e manner to the valve I01 and is adapted to adjust the latter to its open or closed position depending upon the direction to which the motor I09 is energized for rotation. The mechanical connection of the motor I09 to the valve I01 is such as to increase and decrease the supply of heating agent to the furnace 3 as the furnace temperature falls below or rises above a predetermined level.

The motor I09 is energized for rotation in one direction or th other depending upon which of the two opposed field windings is energized by means of a switch IIO. As illustrated in Fig. 1, current fiows from the alternating current supply line L through a conductor III to a switch arm H2 which is insulated from but may be carried by the carriage 23 which carries the potentiometer slidewire contact 1, thence by either of two opposed contacts H3 or II4, conductors H5 or IIS and one field winding or the other field winding of motor I09 to the supply line L. Although not shown the contacts I I3 and I I4 of the switch IIO are made adjustable so that both the control point setting and sensitivity of the apparatus may be set in a manner well known in the art.

In Figs. 4, 5 and 6 I have illustrated, more or less, diagrammatically modifications of the conversion circuit of the arrangement of Fig. 1 for translating the unbalanced potentiometric direct currents into transient oscillating currents which may be readily amplified and also for producing a reference transient oscillating current for energizing the anode circuits of the triodes of valve 53.

The terminals I03 and I04 In the modification of Fig. 4 designate the correspondingly identified output terminals of the filter I6 of Fig. 1. Thus in Fig. 4 as in Fig. 1 the unbalanced potentiometric direct currents are impressed on the terminals I03 and I04. The unbalance potential derived from the potentiometric network I is applied through an inductance I9 across the terminals of a condenser H1. The upper terminal of condenser H1 is connected through a condenser I Il, the primary winding I1 of transformer I0, and contacts 33 and 34 of the interrupter switch to the lower terminal of condenser H1. Th condenser H1 is preferably much larger in capacity than the condenser Ill. The condenser H8 is so selected in relation to the inductance of winding I1, as is the condenser .20 of Fig. 1, as to satisfy the condition of By satisfying this condition, a rapidly vanishing oscillatory discharge current will flow through the local circuit including elements H1, H8, I1 and I4 when a unidirectional potential is impressed upon the condenser II1 upon each engagement and disengagement of the contact members 33 and 34 of the switch I4. The character of this oscillatory discharge current may be exactly the same as the character of the oscillatory discharge current produced by means of the conversion circuit of the arrangement Fig. 1 and illustrated in Fig. 3.

In Fig. 4, the potential tapped off slide wire resistance 10 is impressed on a condenser II9 through inductance 13. The condenser H9 is included in an oscillating circuit which may be traced through the lower terminal of condenser II9 through a condenser I20, the transformer primary winding 11 and the interrupter switch I4 to the upper terminal of condenser I I9. The condenser II! is preferably of much larger capacity than the condenser I20 and the latter is so selected in relation to the inductance of the transformer primary winding 11 as to permit the and conductor I2I to the upper terminal con discharge of an oscillatory current in the circuit including elements I4, H9, I20 and 11 whenever the contact members 33 and 34 of switch I4 move into and out of engagement. The condenser I20 is so selected that the frequency of this oscillatory discharge current will be the same as that established in the transformer primary winding I1.

In the modification disclosed in Fig. 5, the unbalance potentials from the potentiometric network I which appear across the filter output terminals I03 and I04 are impressed on a condenser H1. The potential so produced on the condenser H1 is impressed on an oscillating circuit including the parallel connected transformer primary winding I1 and condenser 20 through a circuit which may be traced from the upper terminal of condenser H1 through inductance I9 to the parallel connected elements I1 and 20 and through a conductor I2I to the lower terminal of condenserlfl.

Similarly, the potential tapped off slide wire resistance 10 is impressed across the terminals of a condenser H9 and the potential produced on condenser I I9 is impressed on the oscillating circuit including the parallel connected transformer primary winding 11 and condenser 19 through a circuit which may be traced from a lower terminal of condenser H1 through inductance 13 to the parallel connected elments 11 and 19 and conductor I2I to the upper terminal of condenser H9. In this modification the contact member 33 of the interrupter switch I4 is connected to the upper terminal of the parallel connected elements l1 and 20 and the contact member 34 is connected to the lower terminal of the parallel connected elements 11 and 19.

When the contact members 33 and 34 are opened, the potential across condenser- H1 is impressed on the parallel connected elements I1 and 20 to cause the establishment of an oscillatory discharge current in the local circuit in cluding elements I1 and 20 and the potential across condenser I I9 is impressed on the parallel connected elements 11 and 19 to cause the establishment of an oscillatory discharge current through the last mentioned elements. When the contact members '33 and 34 are moved into engagement with each other, the upper terminal of elements I1 and 20 is directly connected to the lower terminal of elements 11 and 19. This operation will cause the potentials on the condensers 20 and 19 to equalize. As will be apparent to those skilled in the art the change in potential across the condensers 20 and 19 effects the establishment of an oscillatory discharge current in each of the oscillating circuits. Accordingly, an oscillatory discharge current is established in each of the oscillating circuits upon each engagement and disengagement of the contact members of the switch I4.

In Fig. 6, the unbalanced potentiometric potentials are impressed on the condenser H1 and the potential derived from the slide wire resistance 10 is impressed on the condenser H9. The condenser H1 i connected in an oscillating circuit which may be traced from the upper terminal of condenser H1 through a condenser H8 the primary winding I1 of transformer I8 and conductor I2I to the lower terminal of condenser H1. to an oscillating circuit which may be traced from the lower' terminal of condenser H9 through condenser I20, transformer primary I21 denser H9. The condenser H8 is so chosen in relation to the inductance and resistance of winding I1 as to permit the establishment of an oscillating discharge current through the winding I1 when a potential is impressed on the condenser H1 and likewise the condenser I20 is so chosen in relation to the resistance and inductance of the winding 11 as to permit the production of an oscillatory discharge current through the winding 11 when a potential is impressed on the condenser H9.-

In this modification, the interrupter switch I4 is connected between the left end terminal of the condenser H8 and the left end terminal of condenser I20. the potential established on the condenser H1 by the potentiometric network unbalance causes an oscillatory discharge current through the transformer primary winding I1. Similarly, the potential on condenser H9 then establishes an oscillatory discharge current through the transformer primary winding 11. When the contact members of switch I4 are moved into engagement, the potential across the transformer primary Winding I1 and condenser H8 is equalized to that across the transformer primary winding 11 and the condenser I20. Such equalization also causes the establishment of an oscillatory discharge current in the primary winding I1 and also in the primary winding 11.

While in accordance with the provisions of the statutes, I have illustrated and described the best form' of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that certain features of my invention may sometimes he used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Similarly, the condenser H9 is connected Patent is:

1. 'The method which comprises producing'an electromotive force to be measured having a very low frequency component which is difficult of direct amplification, opposing said electromotive force to a known electromotive force, deriving from the resultant of said electromotive forces a succession of waves of transient oscillatory currents, amplifying said oscillatory currents, and applying the amplified currents to effect a balance between said opposed electromotive forces.

2. The method of balancing a potentiometer to measure a current which is difficult of direct amplification and the variations of which are operative to unbalance said potentiometer to produce an unbalanced or resultant potentiometer current which comprises translating the resultthe resultant of said electromotive forces into a succcession of waves of transient oscillatory currents of predetermined frequency and of one phase or of opposite phase depending upon the When the switch I4 is opened,

polarity of the resultant electromotive force, simultaneously producing an additional succession of waves of transient oscillatory currents of said predetermined frequency and in phase with said first mentioned oscillatory currents when the latter are of said one phase, applying said first mentioned oscillatory currents simultaneously to the input circuits of a pair of electronic valves, applying said additional oscillatory currents to the output circuits of said electronic valves in opposite phase relation, and controlling the magnitude of said known electromotive force in accordance with the difference between the magnitudes of the average current flows in the output circuits of said electronic valves.

4. The method of measuring an unknown unidirectional electromotive force of minute magnitude to eliminate the effects of extraneous fluctuating electromotive forces which are superimposed upon said unidirectional electromotive force which consists of the steps of opposing said electromotive force to a known unidirectional electromotive force, filtering the resultant of said electromotive forces to remove substantially all of the extraneous electromotive forces, detecting and amplifying the extraneous electromotive forces present in the filtered resultant electromotive force and superimposing the amplified quantity on the unfiltered resultant electromotive force in opposition to the extraneous electromotive forces, translating said filtered resultant electromotive force into a succession of waves of rapidly vanishing high frequency oscillatory currents, amplifying said oscillatory currents, and applying the amplified currents to effect a balance between said unknown and known electromotive forces.

5. The method of measuring an unknown electromotive force of minute magnitude to eliminate the effects of extraneous fluctuating electromotive forces which are superimposed upon said unknown electromotive force which consists of the steps of opposing said unknown electromotive force to a known electromotive force, fittering the resultant of said electromotive forces to remove substantially all of the extraneous electromotive forces, detecting and amplifying the extraneous electromotive forces present in the filtered resultant electromotive force and superimposing the amplified quantity on the unfiltered resultant electromotive force in opposition to the extraneous electromotive forces to render said resultant electromotive force substantially free from said extraneous electromotive forces, and applying the filtered resultant electromotive force to effect a balance between said unknown and known electromotive forces.

6. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies difficult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said first mentioned electromotive force a reference electromotive force, means for translating the resultant of said electromotive forces into a succession of waves of transient oscillatory currents, an amplifier associated with said system to amplify said oscillatory currents, and means controlled by the amplified currents to effect a balance of said electromotive forces.

7. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies diflicult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a reference electromotive force, means for translating the resultant of said electromotive forces into transient oscillating currents which may be readily amplified, an amplifier associated with said system to amplify said transient oscillating currents, and means controlled by the amplified transient oscillating currents to effect balance of said electromotive forces,

8. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies difficult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a reference electromotive force, means for translating the resultant of said electromotive forces into a succession of waves of rapidly vanishing high frequency oscillatory discharge currents comprising a circuit on which the resultant of said electromotive forces is applied and having connected in series therein a switch, and capacitive, resistive and inductive components, said switch being operative to periodically open and close said circuit, an amplifier associated with said system to amplify said oscillatory discharge currents, and means controlled by the amplified currents to effect a balance of said electromotive forces.

9. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequency difficult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a reference electromotive force to derive a resultant electromotive force, means for translating the resultant electromotive force into a succession of waves of transient oscillating currents of one phase or of opposite phase depending upon the polarity of said resultant electromotive force, and phase responsive means controlled by said oscillatory currents to effect a balance of said opposed electromotive forces.

10. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies difficult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a variable reference electromotive force, means to vary said reference electromotive force including a reversible electrical motor, a source of electric current for energizing said motor, and means for controlling the application of said energizing current to said motor to selectively actuate the latter for rotation in one direction or the other including means for translating the resultant of said electromotive forces into a succession of waves of transient oscillatory currents of one phase or of inidirectional potential of one polarity or of op- Josite polarity depending upon the phase of said ;ransient oscillatory currents, and means controlled by said unidirectional potential to control the fiow of energizing current from said source to said motor to actuate the latter for rotation in the direction to reduce said resultant electromotive force.

11. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies difiicult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to saidelectromotive force a variable reference electromotive force, means to vary said reference electromotive force including a two phase reversible electrical motor, a source of alternating current for energizing one of the phases of said motor, and means for energizing the other phase of said motor to control the actuation thereof including means for translating the resultant of said electromotive forces into a succession of waves of transient high frequency oscillatory currents of one phase or of opposite phase depending upon the polarity of said resultant electromotive force, phase responsive .means to produce a unidirectional potential of one polarity or of opposite polarity depending upon the phase of said oscillatory currents, means to translate said unidirectional potential into a fluctuating current of the same phase or of opposite phase as said alternating current source depending upon the polarity of said direct current potential and of the same frequency as said alternating current source, and means to apply said fluctuating current to the other phase of said motor.

12. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies difficult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a reference electromotive force, means for translating the resultant of said electromotive forces into a succession of waves of transient oscillatory currents of predetermined frequency and of one phase or of opposite phase depending upon the polarity of said resultant electromotive force and for simultaneously producing an additional succession of waves of transient oscillatory currents of said predetermined frequency and in phase with said first mentioned oscillatory currents when the latter are of said one phase, electronic amplifying means to amplify each of said transient oscillatory currents, a pair of electronic valves each having an input circuit and an output circuit, means to impress said first mentioned oscillatory currents to the input circuits of said valves in parallel, means to impress said additional oscillatory currents to the output circuits of said valves in opposite phase relation, and means responsive to the difference between the magnitudes of the average current flows in the output circuits of said valves to control the magF nitude of said reference electromotive force to reduce said resultant electromotive force.

13. The combination of claim 12 wherein said translating means includes a circuit on which the resultant of said electromotive forces is applied and having connected in series therein, a switch and an impedance comprised of capacitive, resistive and inductive components for producing said first mentioned oscillatory current, a separate circuit having connected in series therein a source of unidirectional potential, said switch, and an impedance comprised of capacitive, resistive and inductive components for producing said additional oscillatory current, and means to periodically open and close said switch, the capacitive, resistive and inductive components of each impedance being so related that the resistive component squared is less than four times the ratio of the inductive component to the capacitive component.

14. The combination of claim 12 wherein said translating means includes a circuit on which the resultant of said electromotive forces is applied and having connected therein an impedance comprised of capacitive, resistive and inductive components for producing said first mentioned oscillatory current, a separate circuit on which a source of unidirectional potential is applied and having connected in series therein an impedance comprised of capacitive, resistive and inductive components .for producing said additional oscillatory current, a connection between said circuits connecting said impedances in series, a switch in said connection, and means to periodically open and close said switch, the capacitive, resistive and inductive components of each impedance being so related that the resistive component squared is less than four times the ratio of the inductive component to the capacitive component.

15. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies diflicult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a reference electromotive force, means for trans lating the resultant of said electromotive forces into a succession of waves of transient high frequency oscillatory currents of one phase of opposite phase depending upon the polarity of said resultant electromotive force and for simultaneously producing two additional successions of waves of rapidly vanishing oscillatory currents, one of 'said additional oscillatory currents being in phase with said first mentioned oscillatory current when the latter is of one phase and the other of said additional oscillatory currents being in phase with the said first mentioned 0scillatory current when the latter is of opposite phase, means to compare the phase of said first mentioned oscillatory current to'the phase of said additional oscillatory currents and arranged to produce a unidirectional potential of one polarity or of opposite polarity depending upon the phase of said first mentioned oscillatory current, a source of alternating current of predetermined frequency, means to translate said unidirectional potential into a fluctuating current of the frequency of said alternating current source and of one phase or of opposite phase depending upon the polarity of said unidirectional potential, and means including an alternating current motor energized by said alternating current source and by said fluctuating current to effect a balance of said opposed electromotive forces.

16 A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctutaions in magntiude with time comprise components of frequencies difficult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a variable reference electromotive force, means to vary said reference electromotive force including a two phase reversible electrical motor, a source of alternating current for energizing one of the phases of said motor, and means for energizing the other phase of said motor to control the actuation of the latter including means for translating the resultant of said electromotive forces into a succession of Waves of rapidly'vanishing high frequency oscillatory currents of one phase or of opposite phase depending upon the polarity of said resultant electromotive force and for simultaneously producing two additional successions of waves of rapidly vanishing oscillatory currents of the same frequency as said first men-- tioned oscillatory currents, one of said additional currents being in phase with said first mentioned oscillatory currents when the latter are of one phase and the other of said additional oscillatory currents being in phase with said first mentioned oscillatory currents when the latter are of the opposite phase, a pair of electronic valves each having an input circuit and an output circuit, means to apply said first mentioned oscillatory current to the input circuits of said valves, means to impress one of said additional oscillatory currents on the output circuit of one of said valves, means to impress the other of said oscillatory currents on the output circuit of the other of said valves, a resistance connected in the output circuits of each of said valves, a condenser connected across each of said resistances whereby a direct current potential is produced across each of said condensers in accordance with the phase and amplitude of said first mentioned transient oscillating current, a second pair of electronic valves each having an anode, a cathode and a control elcetrode, a transformer having a primary winding energized by said alternating current source and also having a center tapped secondary winding, a connection from one end of said secondary winding to the anode of one of said valves and a connection from the other end of said secondary winding to the anode of the other valve, a connection from both of said cathodes to the center tap on said secondary winding including said other phase of said. motor, means to impress the potential produced across one of said condensers between the control electrode and cathode of one of said valves, and means to impress the potential produced across the other of said condensers between the control electrode and cathode of the other of said valves;

1'7. A self balancing system for the measurement of an electromotive force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies diflicult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said electromotive force a variable reference electromotive force, means to vary said reference electromotive force including an alternating current motor having a pair of balanced operating circuits, a pair of electronic valves connected each in one of said circuits, each of said valves having an input circuit and an output circuit, circuit means for supplying alternating current to the output circuits of said electronic 15 valves in opposite phase relation, and means to control the input circuits of said valves including means for translating the resultant of said electromotive forces into a succession of waves of rapidly-vanishing high frequency oscillatory currents of one phase or of opposite phase depend ing upon the polarity of said resultant electromotive force and for simultaneously producing two additional successions of waves of rapidly vanishing oscillating currents of the same frequency as said first mentioned oscillatory currents, one of said additional currents being in phase with said first mentioned oscillatory currents when the latter are of one phase and the other of said additional oscillatory currents being in phase with said first mentioned oscillatory currents when the latter are of the opposite phase, electronic amplifying means to amplify the first mentioned of said oscillatory currents, at second pair of electronic valves each having an input circuit and an output circuit, means to simultaneously apply the amplified components of said first mentioned oscillatory currents to the input circuits of said valves, means to impress one of said additional currents on the output circuit of one of said second mentioned valves and to impress the other of said additional currents on the output circuit of the other of said second mentioned valves, a resistance connected in the output circuits of each of said valves, a condenser connected across each of said resistances whereby a unidirectional potential is produced across each of said condensers in accordance with the phase and amplitude of said first mentioned oscillatory currents, a connection to impress the potential produced across one of said condensers to the input circuit of one of said first mentioned valves, and a connection to impress the potential produced across the other of said condensers to the input circuit of the other of said first mentioned valves.

18. In a measuring instrument, a circuit including a direct current electrical quantity to be measured, means for translating said direct current electrical quantity into a transient oscillating current of one phase or of opposite phase depending upon the polarity of said direct current electrical quantity, means including electronic amplifying means to amplify said transient oscillating current and to produce a unidirectional potential of polarity and magnitude corresponding to the phase and the amplitude of said transient oscillating current, and means to measure the magnitude of said unidirectional potential.

19. In a. measuring instrument, a circuit including a direct current electrical quantity to be measured, means for translating said direct current electrical quantity into a transient oscillating current of one phase or of opposite phase depending upon the polarity of said direct current electrical quantity which may be readily amplified comprising a circuit including a switch, said direct current electrical quantity, and ca.- pacitive, resistive and inductive components-connected in series, said switch being operative to periodically open and close said circuit, electronic amplifying means to amplify said transient oscillating current and to produce a direct current potential of polarity and magnitude corresponding to the phase and the amplitude of said transient oscillating current, and means to measure the magnitude of said direct current potential.

ment of an electromotlve force too small to directly effect control of said system and whose fluctuations in magnitude with time comprise components of frequencies diflicult of direct amplification comprising an electrical circuit upon which said electromotive force isimpressed, means for opposing to said electromotive force a referenceelectromotive force, circuit means for translating the resultant of said electromotive forces into a transient oscillating current of one phase or of opposite phase depending upon the polarity of said resultant electromotive force, means through which the resultant of said electromotive forces is applied to said circuit means to exclude extraneous fluctuating electromotive forces from said circuit means, an amplifier associated with said circuit means to amplify said transient oscillating currents, and means responsive to the phase of said oscillatory currents to effect balance of said electromotive forces.

21. The combination of claim 20 wherein the said means through which the resultant of said electromotive forces is applied to said circuit means includes a filter.

22. The combination of claim 20 wherein the said means through which the resultant of said electromotive forces is applied to said circuit having an input circuit connected to the secondary winding of said air core transformer and having an output circuit, and means controlled by the currents in the output circuit of said ammeans includes a filter and an electronic ampli- 7 of said filter whereby the effects of the extraneous fluctuating electromotive forces passing through the filter are cancelled out.

23. In a measuring instrument, a circuit including a unidirectional electromotive force to be measured, means for translating said unidirectional electromotive force into an oscillating electromotive force of one phase or of opposite phase depending upon the polarity of said unidirectional electromotive force and which may be readily amplified, means through which said unidirectional electromotive force is applied to said translating means including a filter and an electronic amplifier arranged to detect the presence of extraneous fluctuating electromotive forces appearing at the output of said filter and to impress a fluctuating electromotive force of opposite phase on the input of said filter to neutralize the effects of extraneous fluctuating electromotive forces passing through said filter, electronic amplifying means to amplify said oscillating electromotive force and to produce a unidirectional potential of polarity and magnitude corresponding to the phase and the amplitude of said oscillating electromotive force, and means to measure the magnitude of said unidirectional potential.

24. A self balancing system for the measurement'of an electromotive force too small to directly effect control of said system and Whose fluctuations in magnitude with time comprise components of frequencies difilcult of direct amplification comprising an electrical system upon which said electromotive force is impressed, means for opposing to said first mentioned electromotive force a reference electromotive force,

plifier to effect a balance of said electromotive forces.

25. The combination of claim 24 in which the secondary winding of said air core transformer is tuned to the frequency of said transient oscillatory currents.

26. In a measuring instrument, a circuit ineluding a unidirectional electromotive force to be measured, means for translating said unidirectional electromotive force into 'an oscillating electromotive force of phase and amplitude depending upon the polarity and amplitude of said unidirectional electromotive force, means through which said unidirectional electromotive force is applied to said translating means including a filter and an electronic amplifier arranged to detect the presence of extraneous fluctuating electromotive forces appearing at the output of said filter and to impress a fluctuating electromotive force of opposite phase on the'input of said filter to neutralize the effecs of extraneous fluctuating electromotive forcespassing through said filter, said translating means including an air core transformer having a primary winding in which said oscillating electromotive force is produced and having a secondary winding inductively associated with primary winding, electronic amplifying means having an input circuit connected to the secondary winding of said air core transformer to produce a unidirectional potential of polarity and magnitude corresponding to the phase and the amplitude of said oscillating electromotive force, and means to' measure the magnitude of said unidirectional potential.

27. Apparatusfor measuring an unknown electromotive force of minute magnitude and operative to eliminate the effects of extraneous fluctuating electromotive forces which are superimposed upon said electromotive force comprising render said filtered resultant electromotive force substantially free from said extraneous electromotive forces, and means to apply the filtered resultant electromotive force to -effect a balance between said known and unknown electromotive forces.

28. The combination of claim 27 wherein said last mentioned means includes means to translate said filtered resultant electromotive force into a fluctuating electromotive force, an air core transformer having a primary winding on which said fluctuating electromotive force is impressed and having a secondary winding inductively associated with said primary winding, an electronic amplifying means having an input circuit connected to the secondary winding of said transformer and having an output circuit, and means connected tothe output circuit of said electronic amplifying means to effect a balance between said known and unknown electromotive forces.

HARRY S. JONES. 

